1. Field of the Invention
The present invention relates to a method for producing a high-purity anhydrous alkaline metal sulfide. Precisely, it relates to a method for producing an anhydrous alkaline metal sulfide having a reduced impurity content, for which are used an alkaline metal hydroxide and hydrogen sulfide as the starting compounds.
The invention also relates to a method for producing an alkaline metal sulfide solution in which the ratio by mol of the alkaline metal to sulfur ratio is controlled. Precisely, the method, starting from an alkaline metal hydroxide and hydrogen sulfide, comprises forming an alkaline metal sulfide from the starting compounds, followed by partially hydrolyzing the sulfide to give the intended alkaline metal sulfide solution in which the ratio by mol of the alkaline metal to sulfur ratio is controlled.
2. Description of the Related Art
For the starting material in the field of recent engineering plastics and medicines, required are high-purity anhydrous alkaline metal sulfides. For example, in the art, anhydrous sodium sulfide has heretofore been used as the sulfur carrier in producing typical super-engineering plastics, polyarylene sulfides, through polymerization. On the other hand, the present applicant has previously proposed a method of using anhydrous lithium sulfide in producing polyarylene sulfide resins (see Japanese Patent Laid-Open No. 207027/1995). The method is advantageous in that the polyarylene sulfide resins produced are easy to separate from sodium chloride. However, lithium sulfide is expensive, and its industrial scale production is not as yet realized. In that situation, it is desired to establish an inexpensive method of producing high-quality lithium sulfide. For producing lithium sulfide, known are (1) a method of heating simple lithium with sulfur to a temperature not lower than the melting point of the reaction system (see Troost L.; Ann. Chim. Phys., 1857, 51(3), p. 103); (2) a method of reducing lithium sulfate under heat with carbon, hydrogen or ammonia (see K. V. Samsonof and S. V. Dorozdwa; Handbook to Sulfidesxe2x80x94Physical Properties and Phase Diagramsxe2x80x94Moscow Metallurgy Publishing, 1972, Japan-Soviet Press); and (3) a method of reacting an alkaline metal hydroxide with hydrogen sulfide in a non-polar proton solvent to give lithium hydrosulfide followed by processing it for dehydrosulfurization (see the applicant""s Japanese Patent Laid-Open No. 330312/1995).
However, the conditions for the methods (1) and (2) are extremely severe, therefore inevitably requiring complicated and severe production processes. The method (3) is the basis for the present invention, but there still remain some problems with it in point of the probability of stable production of products of high purity and quality. Specifically, in the method (3), the molar ratio of lithium/sulfur (referred to as Li/S) in lithium sulfide produced is 2.10 or more.
On the other hand, in producing polyarylene sulfides having a high molecular weight, used is an anhydrous alkaline metal sulfide, such as anhydrous sodium sulfide, anhydrous lithium sulfide or the like of high purity, as the sulfur carrier. Relating to this, one recent finding is that a solution of an alkaline metal sulfide in an aprotic solvent (hereinafter referred to as xe2x80x9can alkaline metal sulfide solutionxe2x80x9d), in which the alkaline metal-to-sulfur molar ratio in the alkaline metal sulfide is larger than 2.0, for example, between 2.1 and 2.2, is needed for producing high-grade polyarylene sulfides having different physical properties. For this, it is desired to establish a method for producing an alkaline metal sulfide solution in which the molar alkaline metal-to-sulfur molar ratio is controlled in any desired manner.
Up to now, no one could fine a proposal that may directly meet the requirement. Some related proposals are:  less than 1 greater than  a method for controlling the Na/S ratio in sodium sulfide, which comprises adding NaOH or NaSH to the system containing sodium sulfide, or comprises mixing a metered amount of liquefied hydrogen sulfide with an aqueous solution of NaOH (see Japanese Patent Laid-Open No. 285807/1991); and  less than 2 greater than  a method for controlling the Li/S ratio in lithium sulfide being used, as one starting compound, in producing polyarylene sulfides, which comprises bubbling a lithium hydroxide-containing N-methyl-2-pyrrolidone (NMP) solution with hydrogen sulfide being introduced thereinto to thereby remove alkaline metal chlorides and others from the system, followed by further bubbling the system with nitrogen being introduced thereinto under heat, or comprises adding a lithium salt such as lithium hydroxide, lithium N-methylaminobutyrate (LMAB) or the like to the system (see the applicant""s Japanese Patent Laid-Open No. 207027/1995).
However, the method that comprises adding such a sodium salt or a lithium salt to the reaction system is undesirable, as requiring complicated operations. The other method that comprises reacting a metered amount of liquefied hydrogen sulfide with NaOH or LiOH requires pressure equipment, for which the equipment costs are high. In still the method that comprises bubbling the system with nitrogen being introduced thereinto under heat, the sulfur content of the system is controlled. In this, however, lithium hydrosulfide is subjected to dehydrosulfurization, namely, in this, its Li/S ratio is varied from 1 to 2, but not controlled to a desired one.
The invention is to provide a method for producing a high-purity anhydrous alkaline metal sulfide having a reduced content of alkaline metal hydroxides and alkaline metal hydrosulfides, more concretely, having an alkaline metal-to-sulfur molar ratio that falls within the following range:
1.90xe2x89xa6 alkaline metal/sulfur (by mol) xe2x89xa62.10,
the method comprising reacting an alkaline metal hydroxide with hydrogen sulfide in an aprotic organic solvent to give an alkaline metal hydrosulfide, followed by subjecting it to dehydrosulfurization.
The invention is also to provide a method for producing an alkaline metal sulfide solution in which the alkaline metal-to-sulfur molar ratio is controlled, especially to be larger than 2.
The present inventor has analyzed the origins of the impurities to be in lithium sulfide in the prior basic patent application (Japanese Patent Laid-Open No. 330312/1995), and has found the following:
(1) In the system of reaction between an alkaline metal hydroxide and hydrogen sulfide in an aprotic organic solvent, if water formed through the reaction remains therein, an alkaline metal hydroxide is formed through hydrolysis of the following formula (I) or (II) during the reaction for dehydration and dehydrosulfurization:
LISH+H2Oxe2x86x92LiOH+H2Sxe2x80x83xe2x80x83(I),
Li2S+2H2Oxe2x86x922LiOH+H2Sxe2x80x83xe2x80x83(II);
and the alkaline metal hydroxide thus formed will be an impurity in the final product, lithium sulfide, thereby making the ratio Li/S in the final product larger than 2.0;
(2) On the other hand, if non-reacted lithium hydrosulfide remains in the reaction system for dehydrosulfurization to give lithium sulfide according to the following formula (III):
2LiSHxe2x86x92Li2S+H2Sxe2x80x83xe2x80x83(III),
this will also be an impurity in the final product, lithium sulfide, thereby making the ratio Li/S in the final product smaller than 2.0.
Based on these findings, the inventor has completed the present invention.
Specifically, the invention relating to the production of an anhydrous alkaline metal sulfide includes the following:
[1] A method for producing an anhydrous alkaline metal sulfide, which comprises bubbling a solution of an alkaline metal hydroxide in an aprotic organic solvent, with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, then stopping the hydrogen sulfide introduction after the system has come to contain substantially no water remaining therein, and further heating the system for dehydrosulfurization.
[2] A method for producing an anhydrous alkaline metal sulfide, which comprises bubbling a solution of an alkaline metal hydroxide in an aprotic organic solvent, with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, and, after the system has come to contain substantially no water remaining therein, further bubbling the system with an inert gas but not hydrogen sulfide gas being introduced thereinto for dehydrosulfurization.
[3] The method for producing an anhydrous alkaline metal sulfide of above [1] or [2], wherein the solution of an alkaline metal hydroxide in an aprotic organic solvent contains a compound capable of being azeotropic with water.
[4] A method for continuously producing an anhydrous alkaline metal sulfide, which comprises:
(1) continuously feeding a solution of an alkaline metal hydroxide in an aprotic organic solvent into a reactor unit having a stirrer therein and equipped with a distillation tower or column, with the liquid level in the reactor being kept constant;
(2) bubbling the solution in the reactor with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, while the pressure in the reactor is controlled to fall between 2 mmHg abs. and 2.0 kg/cm2G, and the temperature therein to fall between 50 and 250xc2x0 C.;
(3) (i) expelling the side product, water and hydrogen sulfide from the system through distillation, and (ii) optionally recycling at least a part of the expelled hydrogen sulfide for bubbling the system; and
(4) after the system has come to contain substantially no water remaining therein, transferring the solution of the intermediate, alkaline metal hydrosulfide in the aprotic organic solvent into a degassing tank, followed by heating it or further bubbling it with an inert gas being introduced thereinto for dehydrosulfurization.
[5] A method for continuously producing an anhydrous alkaline metal sulfide, which comprises:
(1) continuously feeding a solution of an alkaline metal hydroxide and a compound capable of being azeotropic with water in an aprotic organic solvent into a reactor unit having a stirrer therein and equipped with a distillation tower or column, with the liquid level in the reactor being kept constant;
(2) bubbling the solution in the reactor with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, while the pressure in the reactor is controlled to fall between 2 mmHg abs. and 2.0 kg/cm2G, and the temperature therein to fall between 50 and 250xc2x0 C.;
(3) (i) separating the side product, water, hydrogen sulfide, and the azeotropic compound through distillation, and expelling the side product, water and hydrogen sulfide from the system;
(ii) optionally recycling at least a part of the expelled hydrogen sulfide for bubbling the system; and
(iii) recirculating the azeotropic compound having been condensed and separated from water in the receiver at the top of the distillation tower or column, back into the reactor unit optionally along with a part of a fresh azeotropic compound; and
(4) after the system has come to contain substantially no water remaining therein, transferring the solution of the intermediate, alkaline metal hydrosulfide in the aprotic organic solvent into a degassing tank, followed by heating it or further bubbling it with an inert gas being introduced thereinto for dehydrosulfurization.
In addition, the inventor has studied the problems with the method for producing an alkaline metal sulfide solution noted above, and, as a result, has found that the problems with the method can be solved by hydrolyzing a part of the alkaline metal sulfide formed through the reaction between an alkaline metal hydroxide and hydrogen sulfide. On the basis of this finding, the inventor has further completed the present invention.
Specifically, the invention relating to the production of an alkaline metal sulfide solution includes the following:
[1] A method for producing an alkaline metal sulfide solution with the alkaline metal-to-sulfur molar ratio therein being controlled, which comprises bubbling a solution of an alkaline metal hydroxide in an aprotic organic solvent, with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, then stopping the hydrogen sulfide gas introduction after the system has come to contain substantially no water remaining therein, adding a predetermined amount of water to the system, and further heating the system for dehydrosulfurization with preventing water from flowing out of the system.
[2] A method for producing an alkaline metal sulfide solution with the alkaline metal-to-sulfur molar ratio therein being controlled, which comprises bubbling a solution of an alkaline metal hydroxide in an aprotic organic solvent, with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, then adding a predetermined amount of water to the system after the system has come to contain substantially no water remaining therein, heating the system with preventing water from flowing out of the system, and further bubbling the system with an inert gas but not hydrogen sulfide gas being introduced thereinto for dehydrosulfurization.
[3] A method for producing an alkaline metal sulfide solution with the alkaline metal-to-sulfur molar ratio therein being controlled, which comprises bubbling a solution of an alkaline metal hydroxide in an aprotic organic solvent, with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, then stopping the hydrogen sulfide gas introduction after the water content remaining in the system has reached a predetermined amount, and further heating the system for dehydrosulfurization with preventing water from flowing out of the system.
[4] A method for producing an alkaline metal sulfide isolution with the alkaline metal-to-sulfur molar ratio therein being controlled, which comprises bubbling a solution of an alkaline metal hydroxide in an aprotic organic solvent, with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, and, after the water content remaining in the system has reached a predetermined amount, further heating the system with preventing water from flowing out of the system and bubbling it with an inert gas but not hydrogen sulfide gas being introduced thereinto for dehydrdsulfurization.
[5] The method of any one of above [1] to [4] for producing an alkaline metal sulfide solution with the alkaline metal-to-sulfur molar ratio therein being controlled, wherein the solution of an alkaline metal hydroxide solution in an aprotic organic solvent contains a compound capable of being azeotropic with water.
[6] A method for continuously producing an alkaline metal sulfide solution with the alkaline metal-to-sulfur molar ratio therein being controlled, which comprises:
(1) continuously feeding a solution of an alkaline metal hydroxide and optionally a compound capable of being azeotropic with water in an aprotic organic solvent into a reactor unit having a stirrer therein and equipped with a distillation tower or column, with the liquid level in the reactor being kept constant;
(2) bubbling the solution in the reactor with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, while the pressure in the reactor is controlled to fall between 2 mmHg abs. and 2.0 kg/cm2G, and the temperature therein to fall between 50 and 250xc2x0 C.;
(3) (i) expelling the side product, water and hydrogen sulfide from the system through distillation,
(ii) optionally recycling at least a part of the expelled hydrogen sulfide for bubbling the system; and
(iii) recirculating the azeotropic compound having been condensed and separated from water in the receiver at the top of the distillation tower or column, back into the reactor unit optionally along with a part of a fresh azeotropic compound; and
(4) after the system has come to contain substantially no water remaining therein, stopping the hydrogen sulfide gas introduction, then transferring the liquid from the reactor to a hold tank, adding thereto a predetermined amount of water, stopping expelling water from the system, and subsequently heating the system or bubbling the system with an inert gas being introduced thereinto under heat for dehydrosulfurization.
[7] A method for continuously producing an alkaline metal sulfide solution with the alkaline metal-to-sulfur molar ratio therein being controlled, which comprises:
(1) continuously feeding a solution of an alkaline metal hydroxide and optionally a compound capable of being azeotropic with water in an aprotic organic solvent into a reactor unit having a stirrer therein and equipped with a distillation tower or column, with the liquid level in the reactor being kept constant;
(2) bubbling the solution in the reactor with hydrogen sulfide gas being introduced thereinto under heat for dehydration and dehydrosulfurization, while the pressure in the reactor is controlled to fall between 2 mmHg abs. and 2.0 kg/cm2G, and the temperature therein to fall between 50 and 250xc2x0 C.;
(3) (i) expelling the side product, water and hydrogen sulfide from the system through distillation,
(ii) optionally recycling at least a part of the expelled hydrogen sulfide for bubbling the system; and
(iii) recirculating the azeotropic compound having been condensed and separated from water in the receiver at the top of the distillation tower or column, back into the reactor unit optionally along with a part of a fresh azeotropic compound; and
(4) after the water content remaining in the system has reached a predetermined amount, stopping the hydrogen sulfide gas introduction, then transferring the liquid from the reactor to a hold tank, stopping expelling water from the system, and subsequently heating the system or bubbling the system with an inert gas being introduced thereinto under heat for dehydrosulfurization.